This bit doesn't. multiboot.bin gets installed in ${prefix}/share/.
There is a blob shipped in pc-bios/ so if you do -L $srcdir/pc-bios it
should work. However, if you did -L $objdir/pc-bios where $objdir !=
$srcdir, it would not work without your patch. The same is true for
seabios and the other roms.

Your patch would break the first two though. I'm not sure we have a
great solution.